In a mobile communication system, a user terminal is registered in the network as a unit of a tracking area (TA), a location area (LA), or a routing area (RA), in order to originate and receive a call in any cell. When a user terminal in standby status (idle mode) receives a call, all of the base stations within the tracking area place (originate) the call to the user terminal. For this reason, the user terminal has to perform tracking area update (TAU) with a tracking area management apparatus every time the user terminal changes tracking areas (see TS23.060 ver 6.15.0).
FIG. 1 shows mobile communication system architecture in accordance with LTE (Long Term Evolution) which is under discussion in 3GPP (The 3rd Generation Partnership Project). An apparatus which is an upper node of the base station (eNB) manages tracking areas. This apparatus is also referred to as a MME (Mobile Management Entity). The base station (eNB) belongs to one of tracking areas (TA). When the user terminal (UE) moves from the base station eNB#12 belonging to the tracking area TA#1 to the base station eNB#21 belonging to the tracking area TA#2, the user terminal (UE) performs tracking area update with the MME via the base station eNB#21.